How do I list all schemas in PostgreSQL?

To lists all schemas, use the (ANSI) standard INFORMATION_SCHEMA

select schema_name
from information_schema.schemata;

More details in the manual

alternatively:

select nspname
from pg_catalog.pg_namespace;

More details about pg_catalog in the manual

When using the psql command line, you may list all schema with command \dn.

Connect to the psql command --> psql --u {userName} {DBName} then you can type the below command to check how many schemas are present in the DB

DBName=# \dn

Else you can check the syntax by the below steps easily-

1. After connecting the the DB, press

   DBName=# help
   

You will get the below options:

You are using psql, the command-line interface to PostgreSQL.
Type: \copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit

Then press

DBName=# \?

You will get all the options very easily.